1. Field of the Invention
The present invention relates to a solid-state image pickup device, and particularly to a solid-state image pickup device in which a light-receiving sensor portion has a waveguide provided thereon to increase efficiency at which incident light is focused on the light-receiving sensor portion.
2. Description of the Related Art
In most cases of a CCD (charge-coupled device) or a CMOS (complementary metal-oxide-semiconductor) type solid-state image pickup devices, it has an arrangement in which an on-chip microlens is disposed on a plurality of light-receiving sensor portions composed of photo-diodes, for example, comprising pixels through an insulating layer to bring a focal point of incident light passed through the on-chip microlens near the light-receiving sensor portion to thereby guide light into the light-receiving sensor portion.
However, as the insulating layer progressively increases its thickness in accordance with reduction of a size of a pixel and increase of multilayer interconnection layers, it is unavoidable that the increase of the thickness of the insulating layer will exert a more serious influence upon efficiency at which incident light is focused on the light-receiving sensor portion.
As a method of avoiding such problem, there has been known, in recent years, an arrangement in which an insulating layer has a waveguide located at its position opposing a light-receiving sensor portion to efficiently guide light passed through an on-chip microlens into the light-receiving sensor portion (see cited patent references 1, 2 and 3, for example).
FIG. 1 of the accompanying drawings shows a solid-state image pickup device having such arrangement, for example, an arrangement of a CMOS type solid-state image pickup device.
More specifically, FIG. 1 is a cross-sectional view showing an arrangement of one pixel portion of a CMOS type solid-state image pickup device.
A CMOS type solid-state image pickup device, generally depicted by the reference numeral 30 in FIG. 1, has a semiconductor substrate 31 in which a light-receiving sensor portion 32 is formed at its predetermined position. The semiconductor substrate 31 has formed thereon a silicon oxide film (SiO2 film) 33 having insulating function, surface-protection function or surface planarization function, and the silicon oxide film 33 has formed thereon a silicon nitride film (SiN film) 34 having surface-protection function or function to supply hydrogen to the light-receiving sensor portion 32. Then, the silicon nitride film 34 has formed thereon a non-added silicate glass (NSG film) 35, for example, and the NSG film 35 has formed thereon an interconnection layer 36.
The interconnection layer 36 is composed of three layers of interconnection layers 361, 362 and 363. Each of the interconnection layers 361, 362, 363 has an arrangement in which an interconnection material (for example, Cu) 39 is filled into a groove 38 formed at the predetermined position of an insulating film (for example, SiO2 film) 37.
A film 40 is what might be called a barrier film (for example, SiN film, SiC film) formed between the respective interconnection layers 36 in order to prevent the interconnection material (Cu) 39 from being diffused into the insulating film 37. Although not shown, the barrier film 40 is also formed around the groove 38, for example, in order to prevent the interconnection material 39 from being diffused into the insulating film 37.
A passivation film 41 is formed over the interconnection layer 363 of the uppermost layer through the insulating film 37, and a color filter 43 is formed on the passivation film 41 through a planarization film 42.
The color filter 43 has an on-chip microlens 43 formed at its position corresponding to the light-receiving sensor portion 32.
A waveguide 45 for improving focusing efficiency of incident light is formed from the insulating layer (for example, including one portion of the NSG film 35, the barrier layer 40 and the SiN film 34 in addition to the insulating film 37) to the lower end of the passivation film 41.
The waveguide 45 has an arrangement in which only a side wall 461 of a hole (opening) 46 bored through the insulating layer (for example, including one portion of the NSG film 35, the barrier film 40 and the SiN film 34 in addition to the insulating film 37) is covered with a reflective film 47, a material film (SiO2 film) 48 transparent to visible light being filled into the hole 46.
While a thin film having high reflectivity, for example, Al film, Ag film, Au film, Cu film and W film can be used as the reflective film 47, the Al film is most suitable for the application to the reflective film 47 from the standpoints in which it has much experience in the semiconductor manufacturing process, it can be easily processed and it has high reflectivity.
In the CMOS type solid-state image pickup device 30 having the above-mentioned arrangement, light introduced into the waveguide 45 through the on-chip microlens 44 is guided into the light-receiving sensor portion 32 while it is being reflected by the reflective film 47 that covers the side wall 461 of the hole 46.
Next, a method of manufacturing such CMOS type solid-state image pickup device, in particular, a method of forming the waveguide will be described below with reference to FIGS. 2A to 2F.
In FIGS. 2A to 2F, elements and parts identical to those of FIG. 1 are denoted by the identical reference numerals.
As shown in FIG. 2A, let us start describing first the state in which the light-receiving sensor portion 32 has already been formed at the predetermined position of the semiconductor substrate 31 to receive light incident thereon and in which the respective layers of the three interconnection layers 361, 362, 363 and the insulating film (SiO2 film) 37 have already been formed over the light-receiving sensor portion 32.
Next, a resist film (not shown) is formed on the insulating film (SiO2 film) 37 formed on the interconnection layer 363 of the uppermost layer, and a resist mask 50 with a pattern for use in forming the waveguide 45 is formed by patterning this resist film according to the well-known lithography technique as shown in FIG. 2B.
Thereafter, the insulating layer (for example, including the NSG film 35, the barrier film 40 and the SiN film 34 in addition to the insulating film 37) is etched away from the light-receiving sensor portion 32 through this resist mask 50 by a reactive ion etching method (RIE method), for example.
In consequence, as shown in FIG. 2C, the hole (opening) 46 for forming the waveguide 45 is formed on the light-receiving sensor portion 32 at its corresponding position.
By selecting a reactive gas used in the reactive ion etching method, for example, a certain selection ratio can be maintained between the insulating film (for example, including the NSG film 35, the barrier layer 40 and the SiN film 34 in addition to the insulating film 37) and the SiN film 34, thereby making it possible to prevent the bottom portion of the hole 46 from being extended through the SiN film 34.
Next, the resist mask 50 is removed and a metal film (Al film) 471 serving as the reflective film 47 which will be described later on is deposited on the whole surface including the hole 46. In order to obtain a constant film thickness of the metal film 471, the metal film 471 is deposited by a CVD (chemical vapor deposition) method which can obtain high coating property (high coverage).
Thus, as shown in FIG. 2D, the metal film 471 is deposited on the surface containing the hole 46.
Next, as shown in FIG. 2E, while the metal film 471 formed on the side wall 461 of the hole 46 is being left, other metal film 471 is etched away by the reactive ion etching method (RIE method), for example.
Next, the transparent material film (for example, SiO2 film) 48 is filled into the hole 46 by using a well-known plasma method or a high-density plasma method (HDP method), for example. Alternatively, an SOG (Spin on Glass) or an SOD (Spin on Dielectric) may be filled into the hole 46 by using a coating method.
Then, a planarization treatment is carried out to remove the material film formed other than the inside of the hole 46 with the result that, as shown in FIG. 2F, the waveguide 45 may have the arrangement in which the reflective film 47 is formed on only the side wall 461 of the hole 46, the transparent material film 48 being filled into the hole 46.
Thereafter, the passivation film 41, the planarization film 42 and the color filter 43 are formed on the whole surface containing the insulating film 37 and the SiO2 film filled into the waveguide 45, in that order, and the on-chip microlens 44 is formed on the color filter 43 at its position corresponding to the light-receiving sensor portion 32, that is, on the upper portion of the hole 46.
In this manner, there is formed the CMOS type solid-state image pickup device having the arrangement for improving efficiency at which light is focused on the light-receiving sensor portion 32 as shown in FIG. 1.
[Cited Patent Reference 1]
Japanese laid-open patent application No. 7-45805
[Cited Patent Reference 2]
Japanese laid-open patent application No. 8-139300
[Cited Patent Reference 3]
Japanese laid-open patent application No. 2002-118245.
As described above, as the reflective film 47 that covers the side wall 461 of the hole 46, the Al film is most suitable for the application to the reflective film 47 from the standpoints in which it has much experience in the semiconductor manufacturing process, it can be easily processed and it has high reflectivity.
Then, as described above, in order to obtain the constant film thickness of the Al film, the Al film is deposited by the CVD method that can obtain the high coating property (high coverage).
However, when the Al film is deposited by the CVD method, the following problems arise.
More specifically, when the reflective film 47 having the high reflectivity is obtained, it is effective to deposit the reflective film 47 in the low temperature region as the deposition condition. In the low temperature region, the reflective film 47 is selectively deposited on the surface of the metal film so that it becomes difficult to directly deposit the Al film on the surface of the insulating layer formed on the side wall 461 of the hole 46, that is, the insulating film 37, the NSG film 35, the barrier film 40 and the SiN film 34.
The reason that deposition in the low temperature region becomes the effective deposition condition when the reflective film 47 having the high reflectivity is obtained is that the deposition condition depends upon occurrence and growth of core in the early stage.
More specifically, when the reflective film is deposited in the low temperature region, in the early stage of the deposition process, relatively small cores are produced on the surface of the substrate at high density, and cores will grow around these cores later so that a continuous film is formed in a certain time period. Accordingly, since a particle size of a crystal grain of this continuous film is small and the surface of this continuous film is not so uneven, it can be expected to obtain a reflective film having high reflectivity.
When on the other hand the reflective film is deposited in the high temperature region, although it becomes possible to directly deposit the Al film on the surface of the insulating layer formed on the side wall 461 of the hole 46, core grows in the state of gas phase in the high temperature region so that an Al gas becomes particles to smudge the surface of the insulating layer. Thus, even when the Al film is deposited on the surface of the insulating layer, the coverage is lowered, the reflectivity is lowered by remarkably-uneven portions produced on the film surface, and further an Al film with poor adhesion relative to the surface of the insulating film is formed unavoidably.
More specifically, when the reflective film is deposited in the high temperature region, in accordance with the progress of the film deposition process in the high temperature region, relatively large cores are produced on the surface of the substrate at low density in the early stage of the film deposition, whereafter cores grow around these cores. However, when the growth of cores proceeds to form a continuous film, respective cores grow large as compared with the case in which the reflective film is deposited in the low temperature region. Accordingly, the grain size of this continuous film is large and uneven portions on the film surface become remarkably large, thereby resulting in the reflectivity of the reflective film being lowered.
In addition, although it is considered to deposit the Al film on the surface of the insulating layer formed on the side wall 461 of the hole 46 by using a sputtering method, in this case, although not shown, a coverage (in particular, side wall coverage) is remarkably low as compared with the case in which the Al film is deposited on the surface of the insulating layer by the CVD method. Therefore, in order to obtain a predetermined film thickness on the side wall 461 of the hole 46, the Al film having a film thickness several times to several 10s of times as high as the above film thickness should be deposited on the surface of the substrate 31. As a result, the later work process becomes very difficult or becomes impossible.
In addition, there is a possibility that a predetermined film thickness cannot be obtained on the side wall 461 of the hole 46 due to an overhanging portion which is peculiar to the sputtering method.
Accordingly, it has been proposed to solve the above-mentioned problem by depositing a metal film (underlayer metal film) between the reflective film (Al film) 47 and the side wall 461 of the hole 46 (that is, the insulating film 37) as an underlayer film, for example.
However, when Vb-group element such as Ta (tantalum) that has been so far for use with a suitable film such as a barrier metal film or an adhesion layer on the periodic table or IVb-group element such as Ti (titanium) on the periodic table is deposited as the underlayer metal film, the following problems arise.
More specifically, in the solid-state image pickup device, in order to decrease interface state of the light-receiving sensor portion or in order to suppress a white spot by restoring disorders of crystal lattices, hydrogen contained in a light-shielding film composed of a plasma SiN film or an Al film formed on the light-receiving sensor portion is supplied to the light-receiving sensor portion.
In the case of FIG. 1, hydrogen is supplied from the SiN film 34 formed near the light-receiving sensor portion 32, for example, to the light-receiving sensor portion 32.
However, since the material of the aforementioned Vb-group element on the periodic table or the IVb-group element on the periodic table has properties in which its hydrogen absorption rate is high, the underlayer film made of such material absorbs the hydrogen supplied from the SiN film 34 to the light-receiving sensor portion 32. As. a consequence, the disorders of the above-mentioned interface state and the crystal lattice cannot be improved, and hence properties of the solid-state image pickup device 30 are lowered.
While FIG. 1 shows the case in which the hydrogen is supplied from the SiN film 34 formed near the light-receiving sensor portion 32 to the light-receiving sensor portion 32, such a variant is also possible, in which hydrogen is supplied from a light-shielding film (not shown) or the material film 48 filled into the hole 46 and the like to the light-receiving sensor portion 32.
Even in such case, it is unavoidable that hydrogen is absorbed by the underlayer film.